stems chapter 6. outline external form of a woody twig origin and development of stems tissue...

Stems

Chapter 6

Outline

External Form of a Woody Twig Origin and Development of Stems Tissue Patterns in Stems• Herbaceous Dicotyledonous Stems• Woody Dicotyledonous Stems• Monocotyledonous Stems

Specialized Stems Wood and Its Uses

External Form of A Woody Twig Stem meristem produces shoot system with

branches and leaves

Woody twig consists of axis with attached leaves• Node - area of stem where leaves attach

− Alternate or spiral− Opposite - attached in pairs− Whorled - in groups of 3 or more

• Internode - stem region between nodes• Leaf has flattened blade and usually attached to twig

by petiole

External Form of A Woody Twig Axil - angle between petiole

and stem• Axillary bud located in axil

− Become branches or flowers in flowering plants

− Bud scales protect buds Terminal bud at twig tip• Growth makes twig longer• Number of groups of bud

scale scars tells age of twig Stipules - paired, often leaflike

appendages at base of leaf

External Form of A Woody Twig Deciduous trees and

shrubs (lose all leaves annually) - After leaves fall, have dormant axillary buds with leaf scars below• Bundle scars mark food

and water conducting tissue within leaf scars

Origin and Development of Stems Apical meristem at stem

tip• Increases stem length• Dormant before growing

season begins• Protected by bud scales

and by leaf primordia− Leaf Primordia - tiny

embryonic leaves that develop into mature leaves

Longitudinal section through stem tip

Origin and Development of Stems Apical meristem cells

form 3 primary meristems• Protoderm - gives rise to

epidermis• Procambium - produces

primary xylem and phloem• Ground Meristem -

produces pith and cortex, both composed of parenchyma cells

Longitudinal section through stem tip

Origin and Development of Stems Leaf primordia and bud

primordia develop into mature leaves and buds• Traces branch off from

cylinder of xylem and phloem, and enter leaf or bud− Trace - strand of xylem

and phloem– Each trace leaves gap filled with parenchyma in

cylinder of vascular tissue, forming leaf gap or bud gap

Origin and Development of Stems Narrow band of cells between 1° xylem and 1 °

phloem may become vascular cambium• Vascular cambium produces 2° xylem toward center

and 2° phloem toward surface

Origin and Development of Stems Cork cambium (= phellogen) produces cork cells

with suberin and phelloderm cells• Reduce water loss and protect stem against

injury

• Lenticels - parenchyma cells in cork for exchange of gases

Tissue Patterns in StemsSteles

Stele - central cylinder of 1° xylem, 1° phloem, and pith (if present)• Protostele - solid core, phloem surrounds xylem

− Primitive seed plants, whisk ferns, club mosses and ferns

• Siphonosteles - tubular with pith in center− Common in ferns

• Eusteles - discrete vascular bundles− Flowering plants and conifers

Tissue Patterns in Stems

Cotyledons - seed leaves attached to embryonic stems• Store food needed by young seedling

Dicotyledons (Dicots) - flowering plants developing from seeds with 2 cotyledons

Monocotyledons (Monocots) - flowering plants developing from seeds with single cotyledon

Tissue Patterns in StemsHerbaceous Dicotyledonous Stems

Annuals - plants that die after going from seed to maturity within 1 growing season• Usually green, herbaceous plants

• Most monocots are annuals, but many dicots are also annuals

• Tissues largely primary

Tissue Patterns in StemsHerbaceous Dicotyledonous Stems

• Herbaceous dicots - discrete vascular bundles arranged in cylinder

• Vascular cambium between 1° xylem and 1° phloem– Adds 2° xylem and 2° phloem

Dicot stem

Tissue Patterns in StemsWoody Dicotyledonous Stems

Wood - 2° xylem

Differences in wood:• Vascular cambium and cork cambium active all

year:−Ungrained, uniform wood produced

−Some tropical trees


• If wood produced seasonally:−In spring: Relatively large vessel elements of 2°

xylem produced - Spring Wood

−After spring wood: Fewer, smaller vessel elements in proportion to tracheids and fibers - Summer Wood

−In conifers, vessels and fibers absent Tracheids in spring larger than later in

season


One year’s growth of xylem = Annual Ring • Vascular cambium produces more 2° xylem than

phloem− Bulk of trunk = annual rings of wood

• Indicates age of tree• Indicates climate during tree’s lifetime

Vascular Rays - parenchyma cells functioning in lateral conduction of nutrients and water• Xylem Ray - part of ray within xylem• Phloem Ray - part of ray through phloem


Cross section of young stem with

secondary growth


3-D view of dicot wood


Tyloses - protrusions of adjacent parenchyma cells into conducting cells of xylem• Prevent conduction of

water• Resins, gums, and tannins

accumulate, and darken wood, forming heartwood− Heartwood - older,

darker wood in center − Sapwood - lighter, still-

functioning xylem closest to cambium

Tissue Patterns in Stems Woody Dicotyledonous Stems

Softwood - wood of conifers• No fibers or vessel elements

Hardwood - wood of dicot trees Resin Canals - tubelike canals scattered throughout

xylem and other tissues

• Lined with specialized cells that secrete resin

• Common in conifers• Some tropical flowering

plants – Frankincense

Resin canals in pine

Tissue Patterns in Stems Woody Dicotyledonous Stems

Bark - tissues outside vascular cambium, including 2° phloem• May consist of alternating layers of crushed phloem and

cork

Cross section of

young stem with

secondary growth

Laticifers - ducts found mostly in phloem that have latex-secreting cells• Rubber, chicle (chewing gum), morphine

Tissue Patterns in Stems Monocotyledonous Stems

Monocots stems - no vascular cambium nor cork cambium• No 2° vascular tissues or cork• 1° xylem and phloem in discrete vascular bundles

scattered throughout stem

• Xylem closer to stem center and phloem closer to surface

• Parenchyma (ground tissue) surrounds vascular bundles

Cross section of monocot

stem

Tissue Patterns in Stems Monocotyledonous Stems

Typical monocot vascular bundle:• 2 large vessels with several small vessels• First xylem cells stretch

and collapse– Leave irregularly

shaped air space• Phloem = sieve tubes and

companion cells• Vascular bundle

surrounded by sheath of sclerenchyma

Monocot vascular bundle

Specialized Stems

Rhizomes - horizontal stems that grow below-ground and have long to short internodes• Irises, some grasses, ferns

Runners - horizontal stems that grow above ground and have long internodes• Strawberry

Stolons - produced beneath surface of ground and tend to grow in different directions• Potato

Specialized Stems Tubers - swollen, fleshy,

underground stem• Store food• Potatoes - eyes of potato are

nodes

Bulbs - large buds surrounded by numerous fleshy leaves, with small stem at lower end• Store food• Onions, lilies, hyacinths, tulips

Specialized Stems Corms - resemble bulbs, but

composed almost entirely of stem tissue, with papery leaves• Store food• Crocus and gladiolus

Cladophylls - flattened, leaf-life stems• Greenbriars, some

orchids, prickly pear cactus

Prickly pear cactus

Wood and Its Uses In living tree, 50% of wood weight comes from water

content• Dry part of wood composed of about 60-75% cellulose

and about 15-25% lignin

Density - weight per unit volume

Durability - ability to withstand decay• Tannins and oils repel decay organisms

Knots - bases of lost branches covered by new annual rings produced by cambium

Wood and Its Uses

Wood Products• ½ of U.S. and Canadian wood production used as

lumber, primarily for construction− Sawdust and waste - particle board and pulp− Veneer - thin sheet of desirable wood glued to

cheaper lumber• Pulp - second most widespread use of wood

− Paper, synthetic fibers, plastics, linoleum• In developing countries, ½ of cut timber used for fuel

− Less than 10% in US and Canada

Review

External Form of a Woody Twig Origin and Development of Stems Tissue Patterns in Stems• Herbaceous Dicotyledonous Stems• Woody Dicotyledonous Stems• Monocotyledonous Stems

Specialized Stems Wood and Its Uses

stems chapter 6. outline external form of a woody twig origin and development of stems tissue...

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